Electrical resistor



A ril 28, 1970 Filed Aug. 20', 1968' s. sRoKA 3,509,51 1

ELECTRICAL RESISTOR 2 Sheets-Sheet l FIG. 6

INVENTOR STEVE/V SORO/(A ATTORNEY United States Patent O 3,509,511ELECTRICAL RESISTOR Steven Soroka, Willow Grove, Pa., assignor to TRWInc., Philadelphia, Pa., a corporation of Ohio Filed Aug. 20, 1968, Ser.No. 753,969 Int. Cl. Hlllc 7/00 US. Cl. 338-308 7 Claims ABSTRACT OF THEDISCLOSURE BACKGROUND In the manufacture of discrete electricalresistors of the type comprising a cylindrical substrate of anelectrical insulating material having a film of an electrical resistancematerial coated on the surface thereof, one problem is obtainingresistors of a desired resistance value. Since the resistance value ofsuch a resistor is a function of the width and length of the path of theresistance material film, it has been the practice to obtain a desiredresistance value by adjusting the width and length of the resistancematerial path.

One method generally used for adjusting the resistance value of a filmtype resistor by varying the width and length of the resistance materialpath, a method commonly called spiralling, comprises cutting a finegroove through the resistance film which groove extends helically aroundthe substrate, This cuts the resistance material film into a narrow,long path which extends helically around the substrate so as to increasethe resistance value of the resistor. The width and length of thehelical resistance material path depends on the pitch of the helicalgroove and the number of turns of the groove. By measuring theresistance value of the resistor as the groove is being cut, thespiralling can be stopped when the desired resistance value is reached.Although this method of adjusting the resistance value of a resistorpermits the achievement of very accurate resistance values, it has thedisadvantage that is is difficult to achieve only very small incrementsof adjustment particularly when adjusting resistors having very lowresistance values.

SUMMARY It is an object of the present invention to provide a novelconstruction of a film type electrical resistor.

It is another object of the present invention to provide a novelconstruction of a film type electrical resistor which is easily adjustedas to its resistance value by small increments.

It is a further object of the present invention to provide aconstruction of a film type electrical resistor which can provide aresistor of low resistance value and can be adjusted as to itsresistance value by small increments.

It is a still further object of the present invention to provide anelectrical resistor which includes a film of a resistance materialcoated on the cylindrical surface of a cylindrical substrate and whichis adjusted as to its resistance value by small increments by aplurality of narrow grooves in the resistance film with the groovesbeing arranged in spaced relation along a line extending in a helicalpath around and along the cylindrical substrates.

Other objects will appear hereinafter.

BRIEF DESCRIPTION OF DRAWING For the purpose of illustrating theinvention, there is shown in the drawings forms which are presentlypreferred; it being understood, however, that this invention is notlimited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a perspective view of a resistor of the present invention.

FIGURE 2. is a sectional view taken along line 2-2 of FIGURE 1.

FIGURE 3 is a plan view of the surface of the resistor of FIGURE I laidout fiat.

FIGURE 4 is an end view of an apparatus for forming the resistor of thepresent invention shown in FIGURE 1 in one position of the apparatus.

FIGURE 5 is a view similar to FIGURE 4 but showing another position ofthe apparatus.

FIGURE 6 is a top view of the apparatus shown in FIGURE 4.

FIGURES 7 through 10 inclusive are schematic views of an apparatus forforming a modification of the resistor of the present invention indifferent positions of the apparatus.

FIGURE 11 is a plan view of the surface of the modification of theresistor of the present invention with the surface laid out fiat.

FIGURES 12 through 17 inclusive are views similar to FIGURE 11 of stillfurther modifications of the resistor of the present invention.

DESCRIPTION OF INVENTION Referring initially to FIGURES 1 and 2, theresistor of the present invention is generally designed as 20. Resistor20 comprises a cylindrical substrate 22 of an electrical insulatingmaterial, such as glass, ceramic, or a plastic. A film 24 of anelectrical resistance material is coated on the cylindrical surface ofthe substrate 22. The resistance material of the film 24 may be wellknown resistance material such as carbon or a metal either alone or in abinder, such as a plastic, ceramic, or glass. Termination films 26 of anelectrically conductive metal are coated on the ends of the substrate 22and extend over the cylindrical surface of the substrate so as to beelectrically connected to the resistance film 24. Terminal wires 28 aresecured to the end of the substrate 22 and are electrically connected tothe termination films 26.

To adjust the resistance values of the resistor 20, a plurality ofelongated narrow grooves 30 are cut through the resistance film 24 tothe substrate 22. The grooves 30 are arranged so that they all extendalong a line which extends helically around and along the substrates 22and are spaced apart along this helical line. As shown, the grooves 30are positioned in spaced apart, side-by-side relation along the lengthof the substrate. The resistance value of a film type electricalresistor is equal to the resitivity of the resistance material in ohmsper square times the number of squares of the resistance film. Thenumber of squares of the resistance film is equal to the electricallength of the resistance film divided by the electrical width of theresistance film. As shown in FIGURE 3, which shows the resistance film24 unrolled and laid out fiat, the electrical length of the resistancefilm is the distance between the termination films 26, and the width ofthe resistance film is the circumference of the substrate. The grooves30 in the resistance film 24 provide small area of the resistance filmbetween the grooves which are in essence electrically isolated. Thisdoes not change the electrical length of the resitsance film, but eachof the isolated areas reduces the electrical width of the film so as toincrease the resistance value of the resistor 20. Since the insolatedareas are small, the incerase in resistance created by each of theisolated areas is small. Thus by controlling the number of grooves 30 inthe resistance film 24 in the manner which will be explained, theresistance value of the resistor can be adjusted upwardly in smallincremental steps until a desired resistance value is obtained.

One method of forming the grooves 30 in the resistance film 24 is byspiralling machine of the type shown in United States Letters Patent No.2,724,306 to V. A. Woodell, issued Nov. 22, 1955 entitled AutomaticSpiralling Machine For Making Electrical Resistors" which machine ismolded in the manner shown in FIGURES 4-6 inclusive. In general, thespiralling machine comprises a thin cutting wheel 32 mounted forrotation about its center. The electrical resistor 20 is supportedadjacent the edge of the cutting wheel 32 with the longitudinal axis ofthe resistor being parallel to the axis of rotation of the cuttingwheel. The resistor 20 is supported so that it is rotatable about itsown longitudinal axis and at the s m time movable longitudinally acrossthe edge of the cutt ng wheel. Also, the resistor is movable radially ofthe wheel between a position in which it is engaged by the cutting wheeland a position away from the cutting wheel. A cam plate 34 is fixed in astationary position relative to the periphery of the cutting wheel 32. Acam 36 is mounted to rotate about an axis coincident with the axis ofrotation of the resistor 20 and rotates at the same speed as theresistor. A spring 38 holds the cam 36 in contact with the cam plate 34but allows the axis of the cam to move toward and away from the camplate according to the shape of the cam. The cam 36 is either connecteddirectly to the drive for the resistor or is connected by suitablelinkage to the support for the resistor 20 so that periodic contact ofthe cam 30 with cam plate 34 moves the resistor 20 away from the cuttingwheel 32 in a like periodic fashion. The cam 36 is shaped to have a lowportion and a high portion.

In the use of the spiralling machine to adjust the re sistance value ofthe resistor 20, when the low portion of the cam 36 is facing toward thecam plate 34, the resistor 20 engages the cutting wheel 32 so as to cuta groove in the resistance film 24 (see FIGURE 4). Since the resistor 20is moving longitudinally as well as rotating, groove 30 will followhelical line. When the high portion of the cam 36 comes into contactwith the cam plate 34, the resistor 20 is moved away from the cuttingwheel 32 so as to stop the cutting of the groove 30 (see FIGURE When thecam 36 rotates so that the low portion of the cam again faces the camplate 34, the resistor is brought back into contact with the cuttingwheel 32 to cut a groove 30 in the resistance film 24. Since theresistor is moving longitudinally, the second groove 30 is spacedlongitudinally along the resistor from the first groove 30 and extendsalong the same helical line as the first groove. Thus with eachrevolution of rotation of the resistor 20 and the cam 36 a groove 30 iscut into the resistance film 24 with the grooves 30 being spacedlongitudinally along the resistor and all extending along the samehelical line. As each groove 30 is cut through the resistance film, theresistance value of the resistor 20 is increased by a small incrementfor the reason stated above. By continuously measuring the resistancevalue of the resistor 20 the operation can be stopped when a desiredresistance value is reached.

Instead of using a spiralling machine having a cutting wheel to form thegrooves 30 in the resistance film 24 of the resistor 20, any otherwell-known type of spiralling machine can be used. For example, US.Letters Patent No. 2,743,554 to E. G. Dailey, issued May 1, 1956, entitled Apparatus for Blast Etching Electrical Devices described aspiralling machine which uses a fine blast of sand to cut a helicalgroove in the resistance film of a resistor to adjust the resistancevalue of the resistor This type of spiralling machine can be used tomake the resistor 20 of the present invention by providing a controlwhich properly starts and stops the flow of the sand blast during eachrevolution of rotation of the resistor. Also, U.S. Letters Patent No.2,710,325 to S. A. Johnson, issued June 7, 1955, entiled Method andApparatus for Making Electrical Resistors describes a spiralling machinewhich uses an electric arc to form a helical groove in the resistancefilm of a resistor. This type of spiralling machine can be used to makethe resistor 20 of the present invention by either providing the machinewith the camming means of FIGURES 4-6 to move the resistor into and outof engagement with the arc forming stylus or providing a suitablecontrol for starting and stopping the flow of electrical current to thestylus.

Referring to FIGURE 11 there is shown in flat, rolled out form aresistor 20a of the present invention having two rows of the grooves 30awith all of the grooves extending along the same helical line. By havingtwo rows of the grooves each increment of change in the resistance valueof the resistor created by each groove is smaller so that a desiredresistance value can be obtained with cl ser tolerance. The resistor 20ahaving two rows of grooves 30a can be formed by means of a substantiallyelliptical cam 36a as shown in FIGURES 7-10 inclusive. The ellipticalcam 36:: has two low portions and two high portions. Whenthe lowportions of the cam 36a oppose the cam plate 34, as shown in FIGURES 7and 9, the resistor 20a engages the cutting wheel 32 so as to cut agroove 30a in the resistance film 24a. When the high portions of the cam36a engage the cam plate 34 as shown in FIGURES 8 and 10, the resistor20a is moved away from the cutting when so as to interrupt the cuttingof the grooves. Thus, during each revolution of rotation of the cam 36aand the resistor 20a two grooves 30a are cut into the resistance film24a so as to provide the two rows of the grooves.

Referring to FIGURE 12, there is shown in flat, rolled out form aresistor 20b of the present invention having three rows of grooves 30bin the resistance film 24b. The resistor 20b is formed by providingthree cuts in the resistance 2412 during each revolution of theresistor. This can be achieved by using a cam having three low pointsand three high points. The resistor of the present invention can besimilarly provided with more than three rows of the grooves.

FIGURE 13 shows in fiat, rolled out form a resistor 200 of the presentinvention in which the grooves 30c in the resistance film 240 arearranged in staggered relation along the length of the resistor.However, the grooves 300 are all spaced along a line which extendshelically around and along the resistor. This staggered relation of thegrooves 30c can be obtained by a spiralling machine in which the timingof the cuts is varied during each revolution of rotation of theresistor. For example, if the grooves 30c are being cut by a spirallingmachine such as shown in FIGURES 46, the cam 36 would be rotated againsta suitably contoured cam plate allowing a high point of a cam to strikeincrementally earlier or later each revolution. If the grooves 300 arebeing cut in the resistance film 24c from left to right as viewed inFIG- URE 13, the cam plate would be contoured to have a downwardlytapered upper edge from right to left so that each cut would be madelater each revolution. Instead of contouring the cam plate, thestaggered grooves could be obtained by having the cam rotate at a speeddifferent from the speed of rotation of the resistor.

FIGURE 15 shows a resistor 20d of the present invention having thegrooves 30d in staggered relation similar to the resistor 20c of FIGURE13 except that there are more than one groove cut in the resistance film24d during each revolution of rotation of the resistor. FIG- URE 14shows a resistor 20e of the present invention similar to the resistor20d shown in FIGURE 15 except that the grooves We in the resistance film24s are staggered in one direction over one-half of the length of theresistor and staggered in the opposite direction over the other one-halfof the resistor. This arrangement of the grooves can be obtained byusing a cam having a plurality of high points and low points and eithera suitably contoured cam plate or by suitably varying the speed ofrotation of the cam.

FIGURES 16 and 17 show resistance 20 and 20g respectively of the presentinvention having grooves 30 and 30g in the resistance films 24 and 24grespectively which vary in length. In the resistor 20 shown in FIG- URE16 the grooves 30 are relatively short at the ends of the resistor andgradually increase in length toward the center of the resistor so thatthe grooves at the center of the resistor is of the longest length. Thisprovides a resistor having a varying current density along the length ofthe resistor with the highest current density being at the center of theresistor. This type of resistor can be used as a heater resistor. In theresistor 20g shown in FIGURE 17, the grooves 30g at the ends of theresistor are of the largest length and the length of the groovesdecreases to the shortest grooves at the center of the resistor. Thisprovides a resistor having high current densities at the ends of theresistor so as to provide a uniform heat across the entire length of theresistor. The resistors 20 and 20g can be formed by a spiralling machinewith a cam having a single high-point and a cam plate Which is contouredso that the cam engages the cam plate for varying lengths of time duringeach revolution of the rotation of the resistor.

Although various arrangements of the grooves in the resistor of thepresent invention have been shown, it should be understood that anydesired arrangement of the grooves can be used. However, no matter whatarrangement of the grooves are provided, the grooves in the resistancefilm must be spaced along the length of the resistance film and must bepositioned in spaced relation along a line which extends helicallyaround and along the resistor.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

I claim:

1. An electrical resistor comprising a cylindrical substrate of anelectrical insulating material, a film of an electrical resistancematerial coated on the cylindrical surface of said substrate, and aplurality of discrete unconnected elongated grooves in said resistancefilm, said grooves being spaced along said substrate and extending alonga line which extends helically around and along said substrate, thelength of each groove being less than the circumference of thesubstrate.

2. An electrical resistor in accordance with claim 1 in which thegrooves are arranged in a longitudinal row along the substrate with thegrooves being in spaced apart side-'by-side relation.

3. An electrical resistor in accordance with claim 1 in which thegrooves are in circumferentially staggered relation.

4. An electrical resistor in accordance with claim 1 in which thegrooves are of non-uniform lengths.

5. An electrical resistor comprising a cylindrical substrate of anelectrical insulating material, a film of an electrical resistancematerial coated on the cylindrical surface of said substrate, and aplurality of narrow elongated grooves in said resistance film, saidgrooves being spaced along said substrate and extending along a linewhich extends helically around and along said substrate, said groovesbeing arranged in a plurality of longitudinal rows along the substratewith the grooves in each row being in spaced side-by-side relation.

6. An electrical resistor comprising a cylindrical substrate of anelectrical insulating material, a film of an electrical resistanematerial coated on the cylindrical surface of said substrate, and aplurality of narrow elongated grooves in said resistance film, saidgrooves being spaced along said substrate and extending along a. linewhich extends helically around and along said substrate, the grooves atthe ends of the substrate being the shortest in length and the length ofthe grooves progressively increasing to a groove of longest length atsubstantially the center of the substrate.

7. An electrical resistor comprising a cylindrical substrate of anelectrical insulating material, a film of an electrical resistancematerial coated on the cylindrical surface of said substrate, and aplurality of narrow elongated grooves in said resistance film, saidgrooves being spaced along said substrate and extending along a linewhich extends helically around and along said substrate, the grooves atthe ends of the substrate being the longest in length and the length ofthe grooves progressively decreasing to a groove of the shortest lengthat substantially the center of the substrate.

References Cited UNITED STATES PATENTS

